1. Field of the Invention
This invention lies in the field of radio synthesizers and, more particularly, detectors for determining when a phase lock loop synthesizer is out of lock.
2. Description of the Prior Art
The circuit of this invention finds particular use in connection with a phase lock loop synthesizer utilized within an aircraft radio transceiver. The assignee of this invention has recently designed an avionics transceiver to meet the 720 channel selectivity requirement, which transceiver utilizes a synthesizer module employing the latest digital techniques to assure accurate lock-on. While lock-on is quickly and efficiently achieved with this apparatus, there is a period of search following each selection of a new frequency, during which search period the oscillator is being tuned to the newly selected frequency. During this search period when the loop is out of lock, the oscillator output does not correspond to any desired channel frequency, and it is during this period that the detector of this invention inhibits transmission from the oscillator through to either the receiver or transmitter portion of the apparatus, so that extraneous communication is avoided. As soon as the loop is in lock, assuring that the oscillator has been tuned to precisely the desired frequency corresponding to a selected channel, the output circuit is enabled so that the transceiver function proceeds.